Overhead power lines are the structures traditionally used in electric power transmission and distribution to transmit high-voltage alternate current (HVAC) electric energy along long distances. The cables in overhead power lines essentially consist of one or more metallic conductors (usually three or four) suspended by towers. In overhead power lines, the insulation of the conductors is provided by air.
Throughout the present description, the term “high-voltage” refers to a voltage above 30 kV.
HVAC electric energy can also be transmitted by underground power lines.
Underground power lines can be an attracting alternative to overhead power lines for several reasons, such as, for example, landscape aesthetics, abatement of emissions of electromagnetic fields into the surrounding area, and reduction of risk of damage caused by storms, high winds, ice, snow storms, falling trees and so on.
Unlike in overhead power lines, in underground power lines air does not provide for insulation and, hence, the metallic conductor must be otherwise insulated.
Generally, HVAC cables for underground power lines are provided with a metallic electric conductor (usually aluminium or copper) surrounded—from the radially innermost layer to the radially outermost layer—with an inner semiconductive layer, an insulating layer and an outer semiconductive layer. Such layers are usually made of polymeric material. Because of the presence of an insulating layer, the cables for underground power lines are known as “insulated cables”.
Due to the presence of said insulating layer, HVAC insulated cables have a capacitance higher than that of cables used in overhead lines where the insulation is provided by the surrounding air.
As from Gagari D., International Journal of Electrical and Computer Engineering (IJECE) Vol. 2, No. 4, August 2012, pp. 447˜451, the higher capacitance promotes, in an electric power transmission line, the so called “Ferranti effect”, i.e. in an AC electric power transmission line the receiving end voltage is greater than the sending end voltage. This effect is more pronounced as the longer the line and the higher the voltage applied, and gives place to undesirable temporary overloads.
As reported, for example, in “Technical Report on using EHV cables as alternatives to Overhead Lines”, 2009, Tokyo Electric Power Company, even by compensating for the capacitance at each end of the cable, the voltage somewhere in the centre of the cable can become unacceptably high, which will prematurely age the cable insulation amongst other things. Compensation for the cable's capacitance takes the form of large coils or reactors which connect the cable cores to earth. For example, a 400 kV HVAC underground power line about 160 km long, required 24 reactors to fully compensate the cable capacitance.
The Applicant faced the problem to provide low capacitance HVAC insulated cables for power transmission and distribution.
It is known that low capacitance insulated cables can be obtained by increasing the insulation thickness. However, this solution has several drawbacks, such as reduction of flexibility, increase of cable weight, reduction of cable length that can be transported, thus requiring more joints, and an overall increase of manufacturing and deployment costs.
It is also known that low capacitance insulated cables can be obtained by lowering the dielectric constant (or electric permittivity, ∈r) of the polymeric material that is used to form the insulating layer.
The dielectric constant (∈r) of a polymeric material is known to be directly proportional to its polarity and density.
With regard to the polarity, special polymers, for example fluorinated polymers, have a polarity and, as a consequence, a dielectric constant lower than polyolefin polymers generally employed as base material for a HVAC insulating layer. However, such special polymers are notably more expensive than polyolefin polymers and their use is disadvantageous from the economical point of view.
With regard to the density, it can be decreased by expanding the polymeric material. However, an expansion inevitably creates voids and microvoids in the insulating layer, which are at the origin of partial discharge phenomena.
As from, for example, “The Propagation of Partial Discharge Pulses in a High Voltage Cable”, ZZ. Liu et al., 1999, in Proc. Of AUPEC/EECON eds, September 26-29; Darwin, Australia, Northern Territory, Australia, pp. 287-292, partial discharge (PD) activity in high voltage cables is caused by various defects, such as voids. Gas-filled cavities or voids are formed in solid insulation during manufacture, installation or operation. When the electric stress in the void exceeds the breakdown strength of gas within the void, partial discharges will occur. PDs will gradually degrade and erode the dielectric materials, eventually leading to final breakdown.
Partial discharge phenomena assume particular relevance in alternating current transmission and distribution because of the continuous reversal of the electric charge.
Thus, a prejudice exists in the art with regard to decrease the density of the insulating layer by creating voids, or even microvoids, especially in cables for high-voltage alternating current power lines.
U.S. Pat. No. 6,759,595 deals with outdoor termination for a high voltage cable, comprising an insulator body for receiving the high voltage cable, a filling compound provided within the insulator body. Hollow “micro-spheres” filled with gas can be provided in the filling compound to compensate volume, for example during temperature changes. The document states that the gas in the hollow cavities in an insulating medium presents a higher risk of partial discharge.
GB 2 209 167 relates to a composite material of low dielectric constant, having electric properties improved by incorporation of minute hollow spheres in a fibrous polytetrafluoroethylene (PTFE). The formed article of mixture is sintered. When the composite material is used as an electric insulating material, it is desirable from the standpoint of effect of incorporation to select the amount of hollow spheres approximately in the range of from 50 wt % to 95 wt % based on the amount of the composite material.
EP 1 639 608 discloses high voltage insulating materials in solid and liquid form, which are provided in particular for use in high voltage generators for example for radio-technology and computer tomography. The high voltage insulating component is hard and foam-like, and comprises a polymer matrix and a filler, wherein the filler is formed by hollow spheres, wherein the hollow spheres are made of a further material and are filled with a gas. The hollow spheres may be made for example of glass. Hollow spheres preferably have a diameter of for example up to about 100 μm. The dielectric constant of the insulating material may be reduced further the greater the fraction of gas in the insulating material. This fraction increases as the number and diameter of the hollow spheres increase. The insulating materials are produced in the form of high power injection molded parts.
The inclusion of hollow sphere or particles into an insulating component should also take into account the process for manufacturing thereof. While the insulation of a discrete end-product is typically produced by moulding, the insulating layer of a continuous end-product, like a cable, is produced by extrusion where shear and pressure stress can be challenging for some material.